A large quantity of oil worldwide is located in heavy oil and bitumen containing formations. Not including hydrocarbons in oil shale, it has been estimated that there are 1.3 to 1.5 trillion cubic meters (8-9 trillion barrels) of heavy oil and bitumen in-place worldwide. Heavy oil and bitumen containing formations may occur from the surface of the earth to a depth of more than 2000 meters. Heavy oil or bitumen in such formations at a depth of 75 meters or greater may be recovered by in situ extraction wherein wells are drilled into the formation to extract the oil.
In situ extraction of oil from heavy oil or bitumen containing formations is typically impeded by the viscosity of the heavy oil or bitumen. Generally, the viscosity of oil in a heavy oil or bitumen containing formation is sufficiently great that the oil does not easily flow to a well for production.
Thermal methods have been provided for in situ extraction of oil from a heavy oil or bitumen containing formation wherein the viscosity of the oil in the formation is reduced by heating the oil in the formation, thereby mobilizing the oil in the formation for production from the formation via a well. Steam has been used to provide heat in some thermal methods for reducing the viscosity of oil in heavy oil or bitumen containing formations. Steam assisted gravity drainage (SAGD), cyclic steam stimulation (CSS), and vertical steam drive (VSD) are common thermal methods utilized for reducing the viscosity of heavy oil or bitumen in a formation by heating the formation with steam that is injected into the formation.
Solvents have been used in combination with steam to enhance mobilization of oil in a heavy oil or bitumen containing formation for production from the formation. Low molecular weight hydrocarbons have been utilized in combination with steam to reduce the viscosity of bitumen in situ and enhance recovery of hydrocarbons from a bitumen-containing formation. For example, U.S. Pat. No. 6,662,872 provides a process for reducing the viscosity of bitumen in a formation and enhancing recovery thereof by co-injecting steam and a C1-C8 normal hydrocarbon (e.g. methane, ethane, propane, butane, pentane, hexane, heptane, or octane), and U.S. Pat. No. 6,708,759 provides a process for reducing the viscosity of bitumen in a formation and enhancing recovery thereof that, in part, requires co-injecting steam and liquid petroleum condensate comprised of low molecular weight paraffinic hydrocarbons. Higher molecular weight hydrocarbons including aromatic hydrocarbons have also been used in combination with steam to recover heavy viscous oils such as bitumen from an oil-bearing formation. For example, U.S. Pat. No. 4,280,559 discloses a process in which steam is injected into a viscous oil-bearing formation, hydrocarbons are recovered, then a hydrocarbon solvent containing a low concentration of low molecular weight paraffinic hydrocarbons, preferably being light naphtha, gasoline, and or aromatic solvents including benzene, toluene, or xylene is injected into the formation followed by a second steam injection, and hydrocarbons are then recovered from the formation. U.S. Pat. No. 3,838,738 further discloses the use of aromatic hydrocarbons such as benzene and toluene in combination with steam to recover bitumen from a bitumen-containing formation by injecting steam and the aromatic hydrocarbons in a flow path in the formation between an injection well and a production well, where the aromatic hydrocarbons vaporize in the flow path, and the vaporized hydrocarbons condense and mix with the bitumen thereby mobilizing the bitumen for production from the formation through the production well. Other solvents such as carbon disulfide or halogenated hydrocarbons have also been used in combination with steam to mobilize viscous, heavy oils such as bitumen in situ for production from a heavy-oil bearing formation. U.S. Pat. No. 3,838,738 further discloses that carbon disulfide may be used together with steam to mobilize bitumen for production from a bitumen-containing formation, and U.S. Pat. No. 3,822,748 discloses that carbon disulfide or halogenated hydrocarbons may be used as petroleum miscible fluids together with steam at a steam temperature of less than 121° C. (250° F.) to mobilize bitumen for production from a tar sand formation.
The solvents that have been utilized in combination with steam to mobilize viscous oil for production, however, each have attendant difficulties when used to recover oil from a heavy oil or bitumen containing formation. A portion of heavy oil or bitumen, in particular the asphaltene fraction of the heavy oil or bitumen, is not soluble in low molecular weight hydrocarbons, particularly paraffins and most particularly normal paraffins. or paraffinic low molecular weight hydrocarbons. As a result, utilization of a low molecular weight hydrocarbon solvent in combination with steam to extract oil from a heavy oil or bitumen containing formation 1) leaves a substantial fraction of the oil that is not soluble in the solvent in place in the formation, reducing yield; and 2) potentially blocks mobilization of producible oil in the formation by precipitating asphaltenes within the formation. Heavier hydrocarbons such as aromatic hydrocarbons, light naphtha, and gasoline are more miscible with heavy oil and bitumen and do not precipitate asphaltenes from heavy oil or bitumen when used as a solvent in conjunction with steam to extract oil from a heavy oil or bitumen containing formation, however, due to their higher vaporization temperatures, a substantial amount of energy is required to separate these solvents from the produced oil. Economically, use of either the light molecular weight hydrocarbons or the heavier hydrocarbons as a solvent for use in combination with steam for in situ mobilization and recovery of oil from a heavy oil or bitumen containing formation is not practical since the cost and availability of the solvent, particularly heavier hydrocarbons, is prohibitive.
Carbon disulfide, while miscible with heavy oil or bitumen, is not particularly useful in combination with steam for use in situ for recovery of oil from a heavy oil or bitumen containing formation since 1) carbon disulfide is easily hydrolyzed in the presence of steam to form hydrogen sulfide and carbon dioxide, thereby souring and acidifying the formation and 2) is more dense than oil and water, and falls to the bottom of the formation. Halogenated hydrocarbons, also miscible with heavy oil and bitumen, are not particularly useful in combination with steam for in situ recovery of oil from a heavy oil or bitumen containing formation since halogenated hydrocarbons poison oil hydroprocessing catalysts and, therefore, must be completely removed from recovered oil prior to processing the recovered oil and because halogenated hydrocarbons are mutagenic.
It is desirable, therefore, to provide an improved process and a system for recovering oil from a heavy oil or bitumen containing formation.